A semiconductor device includes an oxide insulating layer, an oxide semiconductor layer on the oxide insulating layer, a gate insulating layer on and in contact with the oxide semiconductor layer, and a gate electrode on the gate insulating layer. The oxide semiconductor layer includes a channel region overlapping the gate electrode, and source and drain regions that do not overlap the gate electrode. At an interface between the source and drain regions and the gate insulating layer, a concentration of an impurity on a surface of at least one of the source and drain regions is greater than or equal to 1×1019 cm−3.